1. Field of the Invention
The present invention relates to a printing element substrate, a printhead, and a printing apparatus.
2. Description of the Related Art
Inkjet printing apparatuses described in Japanese Patent Laid-Open Nos. 2002-355970 and 2010-155452 each include a printhead for executing printing on a printing medium. The printhead includes a printing element substrate. The printing element substrate includes a printing element and a drive circuit including a drive transistor for driving the printing element. A power supply line for supplying power to the printing element is isolated from the power supply line of the drive circuit. The drive transistor is arranged between the printing element and the power supply line for supplying power to the printing element.
The printing element substrate described in Japanese Patent Laid-Open No. 2002-355970 controls a voltage to be applied to the printing element by the voltage of the control terminal of the drive transistor. Even if potential fluctuations occur in the power supply line for supplying power to the printing element, this arrangement reduces the influence of the potential fluctuations on the voltage to be applied to the printing element.
When, for example, the printhead is not appropriately mounted, no power supply voltage may be supplied to the power supply line for supplying power to the printing element while a power supply voltage is supplied to the power supply line of the drive circuit.
In this case, since the power supply voltage is supplied to the drive circuit, the drive circuit can output a predetermined voltage to the gate of the drive transistor. On the other hand, since no power supply voltage is supplied to the power supply line for supplying power to the printing element, the drain potential of the drive transistor becomes indefinite. When, for example, the drain potential is 0 [V], the channel potential can also become 0 [V]. Therefore, an overvoltage may be generated between the substrate and the gate of the drive transistor, thereby causing an insulation breakdown.